The field of invention relates to thin film processing and, more specifically, to the field of processing techniques for semiconductor integrated circuits employing copper based metallization technology.
In the manufacture of advanced semiconductor devices, copper (Cu) is beginning to replace aluminum (Al) as the material for metallization. Cu has become desirable due to its lower resistivity and significantly improved electromigration lifetime, when compared to Al.
A problem with Cu based metallurgy, however, involves the rate at which native oxides form on exposed regions of copper. In a free air ambient, it is reported that copper oxides grow 20 Angstroms (xc3x85) within the first minute; while within water environments copper oxides grow 50 xc3x85 within the first minute. Frequently these native oxides are undesirable. That is, as the copper region is typically used to electrically interconnect various devices within an electrical circuit, the formation of the oxide represents an undesirable reduction of conductance. Thus, cleaning, ambient control and/or processing approaches unique to copper metallurgy based manufacturing processes are typically used to control mitigate, or at least limit, the effects of the oxide growth.
One particular problem area concerns bonding pads used for input/output (I/O) connections from the semiconductor chip to its associated package. Specifically, copper based metallization technologies having copper bonding pads experience bond quality problems which are caused by the growth of a relatively thick native oxide on the copper bond pad.
Typically, a wire is bonded as shown in FIG. 1. Referring to FIG. 1a, a capillary 101a threaded with a (typically Gold (Au) or Al) wire 104a having a ball 102a formed at the capillary tip 105a is centered over a chip""s bond pad 103a and (referring now to FIG. 1b) pressed against the face of the bond pad 103b. Then, typically, either thermosonic or thermocompressive energy is applied at the capillary tip 105b to adhere the ball 102b to the bond pad 103b. As shown in FIGS. 1c and 1d, after the ball 102c is adhered to the bond pad 103c, the capillary 101c moves to a package lead 106 where the wire 104d is subsequently wedge bonded to the package lead 106.
Unfortunately, in standard manufacturing processes, a native oxide (not shown in FIG. 1a) forms over the copper bond pad 103a before the ball 102a makes contact with the pad 103a. The native oxide prevents a bond from forming between the wire and the underlying copper pad. Good bonds typically exhibit an intermetallic layer between the ball 102 and the bond pad 103.
As discussed, the native oxide is formed prior to the application of the ball 102 to the bond pad 103. Standard manufacturing processes typically form the bond pad structure immediately following the last (or highest) metallization layer of the interconnect structure of the semiconductor wafer. After the last metallization layer is formed, the metal layer is polished (e.g., by a Chemical Mechanical Polish (CMP)) to a specified thickness range.
After polishing, the semiconductor wafer is cleaned to remove unwanted particulates from the surface of the wafer, most of which result from the prior polishing steps. Native bond pad oxides formed after polishing and before cleaning may be removed by the cleaning process depending on the cleaning chemistry used. After cleaning, the semiconductor wafers are dried, tested and diced. Each individual die is then placed in a chip carrier and epoxied at elevated temperature to a die package before the wire bonding procedure commences.
The entire xe2x80x9cpost cleaningxe2x80x9d process described above results in copper bond pad exposure to an oxygen based ambient/environment at room temperature as well as in an a thermal curing oven for substantial periods of time. As a result, it is difficult to prevent native oxide growth on the exposed bond pads without introducing costly complications to the standard manufacturing sequence just described (for example, one approach is to plasma etch and then deposit a relatively thick layer of another metal film, such as Ni; this would then be followed by a second deposition of palladium and/or gold).
Thus, a cost effective way of controlling the native oxide growth on the copper bond pads is needed.
A method and apparatus are described for removing an oxide from a surface and then commencing application of a passivation layer to the surface within 5 seconds of the oxide removal. The surface may be a copper surface which may further comprise a bonding pad surface. Removing the oxide may further comprise applying a solution comprising citric acid or hydrochloric acid. Applying the passivation layer may further comprise applying a solution comprising a member of the azole family where the azole family member may further comprise BTA. The method may also further comprise completely applying the passivation layer 35 seconds after commencing its application.